In plasma arc cutting technology, the quality of a cut can be described in terms of the dimensional accuracy of the cut parts, cut angle (degree of squareness of the cut face in the direction normal to the cut), the amount of dross on the bottom of the workpiece (which will usually be metal plate), amount of spatter on the top of the workpiece and sharpness of the top and bottom edges of the cut part. Cut quality is determined, in particular, by the effectiveness of metal melting and removal from the workpiece, which depends on factors including the thermal energy delivered to the workpiece and on the momentum of the plasma jet.
The thermal energy delivered to the workpiece depends on the electrical energy of the plasma arc and on the efficiency of energy transfer to the workpiece. If it is assumed that the energy transfer efficiency is substantially constant, then the electrical energy of the plasma arc can be taken as a measure of the thermal energy delivered to the workpiece, which is the approach adopted in the following description.
The amount of electrical energy per unit length of cut is a significant process variable affecting the material melting process. This process variable is determined by the cutting speed, arc voltage, arc current and pressure of the plasma forming gas. Conventional process control for plasma arc cutting relies on regulation of the cutting speed, arc voltage, arc current and plasma gas pressure around constant (optimal) set points which are chosen to ensure the best cut quality for a given plate. In general, the optimal cutting speed cannot be maintained at all times, for example, for a plasma arc cutting operation that is integrated with a manufacturing process such as welding, as in continuous pipe making, the optimal welding speed may determine the use of a non-optimal cutting speed. Also the optimal cutting speed cannot be maintained during the cutting of complex parts using profiling machines because of the finite acceleration capabilities of these machines. That is, the deceleration along the x-axis and acceleration along the y-axis of such a machine during the traversal of a 90.degree. corner results in a decrease of the cutting speed near the corner.
The effect of variations in the cutting speed on the amount of energy per unit length of a cut is twofold. First, the amount of energy per unit length of cut increase with decreasing cutting speed for constant arc voltage, arc current and pressure in the nozzle chamber. Second, the arc voltage increases with decreasing cutting speed thus further contributing to the increase in the amount of energy per unit length of cut. Such an increase in the arc voltage is due to an effective increase in the length of the arc caused by the movement of the arc anode root down the cutting front at reduced cutting speed.
The increase in the amount of energy per unit length of cut when the cutting speed reduces results in an excessive amount of molten metal which cannot be completely removed by the momentum of the plasma jet. Further, at low cutting speeds the shape of the cut front changes resulting in a change in the direction of ejection of molten metal. This leads to dross formation and possibly to corner undercut. Dross is often formed well beyond the deceleration-acceleration region of a corner which is due to the shape of the cut front. Since the cut front depends upon the diffusion of heat through the plate, there is a time dependent mechanism associated with dross formation initiated in the vicinity of the profile corner. This means that a significant part of the profile may be affected by dross formed at the bottom of the plate.
In the prior art the amount of energy per unit length of cut has been controlled by varying the torch-to-workpiece distance. However the amount of variation in this distance that is available and its effect on the amount of energy is generally insufficient to eliminate dross formation in the vicinity of profile corners. The amount of energy per unit length of cut may also be controlled by varying the arc current in response to changes in the cutting speed, for example, by decreasing the arc current while decreasing cutting speed. This type of control of the amount of energy per unit length of cut may ensure effective metal melting, however the cut quality also depends on the effectiveness of metal removal from the workpiece, which in turn depends on the momentum of the plasma jet.
It can be shown that the momentum of a plasma arc jet emanating from the nozzle of a plasma arc cutting torch is approximately proportional to the gas pressure in the nozzle chamber and that there is a strong relationship between this pressure and arc current. Thus the pressure in the nozzle chamber, and therefore the plasma jet momentum, varies with current. The effect of this is that although the energy per unit length of cut could be controlled effectively by varying the arc current, this is at the expense of the plasma jet momentum. That is, a decrease in arc current results in a decrease in the momentum of the plasma jet and this reduces the effectiveness of metal removal by the plasma jet. Thus effective metal removal and therefore a high quality of cut around a profile cannot be maintained.
Japanese Patent No. 1884596 (Kokai 61-262464) by S Hagihara et al, discloses pulsing of the arc current to reduce the amount of dross and to enable high speed cutting. Soviet patent document SU-1632670-A, also discloses pulsing of the arc current to increase the cutting speed. However in both of these disclosures the arc pulsing parameters are fixed during cutting. Japanese Kokoku 44-29967 also discloses pulsing of the arc current, but with a cyclic variation in the amplitude of the pulses to uniformly distribute the heat of the plasma arc down the depth of the cutting groove to reduce narrowing and irregularities in this groove.
Although the above disclosures for pulsing the arc current give improved cut quality, the uniformity over the length of a cut, particularly when producing parts having a complex profile, is variable. Also in the above Japanese and Soviet patent documents, the current pulsing is "upwards" in the sense that the torch is operated at or near its DC rating and current pulses of increasing amplitude are imposed on this DC current such that generally the current rating of the nozzle is momentarily (i.e. for the duration of each pulse or over a lesser period) and repeatedly exceeded. This can lead to problems such as double arcing and thus a shortened lifetime for the torch consumables.